Vu Van Dan, 54, was one of the first people to arrive at the crash site in waters between Quang Ninh Province and Hai Phong City on Wednesday evening.
Dan said he and several fishermen saw the helicopter circling some seaside hills before making "a croaking sound like some part of it was slipping off."
"The helicopter then flew closely past a fishing boat, plunged into the water and exploded. Debris was thrown all over the place. Someone rose out of the water before drowning," he said.
His son Vu Van Truong, who was fishing 50 meters from the crash site, also said the helicopter exploded after plunging halfway into the water, at which point flames rose into the air.
The father and son called other fishermen to help rescue the victims. They arrived after five minutes on speedboats and found a man floating in a life vest.
"We struggled for a while before managing to pull him out of the water. We checked if he was still breathing, but there was nothing. His face had been deformed," Dan said.
The fishermen then turned on their fishing lights and found another female victim, also dead with severe head injuries. They continued the search until midnight, joining forces from the army, police, and coast guard.
The Bell 505 helicopter belonging to the state-owned Northern Vietnam Helicopter Company was guiding the tourists on an aerial tour of Ha Long Bay. The aircraft took off from Tuan Chau Island at 4:56 p.m.
Ground control lost the helicopter’s radio signal at 5:15 p.m.
Four of the five victims’ bodies had been found by Thursday night, including that of the pilot, Colonel Chu Quang Minh, 59.
The tourists have been identified as Ho Ta Luc, 59, his wife Nguyen Thi Hoi, 56, his sister Ho Thi Oanh, 51, and a family friend Pham Thi Be, 60.
The trip was to celebrate Luc's birthday, the victims' family said.
Colonel General Nguyen Tan Cuong, chief advisor of Vietnam People's Army and Deputy Minister of National Defense, said reports filed so far showed that the helicopter had encountered some technical issue.
Cuong said the pilot had tried to turn back to a safe area, "but things happened too fast and the regrettable incident occurred."